This invention relates generally to fuel injectors utilizing check valves, and more particularly to micrometering or varying fuel injection rates using a solid state motor to lift a check valve.
Over time, engineers have come to recognize that undesirable exhaust emissions can be reduced by having the ability to produce at least three different fuel injection rate shapes across the operating range of a given engine. These rate shapes include a ramp, a boot shape, and square fuel injection profiles. In addition to these rate shapes, there is often a need for the injector to have the ability to produce split injections in order to further improve combustion efficiency at some operating conditions, such as at idle.
Although there exist a wide variety of mechanisms for pressurizing fuel in fuel injection systems, almost all fuel injectors include a spring biased needle check valve to open and close the nozzle outlet. In almost all fuel injectors, the needle valve member is only stoppable at two different positions: fully open or fully closed. Because the needle valve members in these fuel injectors are not normally stoppable at a partially open position, fuel injection mass flow can usually be controlled only through changes in fuel pressure.
Hydraulic bias control of the check valve is also possible, such as taught in U.S. Pat. No. 6,024,296 to Wear et al. Another approach is dual nozzle design, but this is an expensive solution.
It would be advantageous to have a reliable mechanism for accurately varying check lift for rate shaping purposes. For example, reducing maximum lift of the check valve member could provide pre-metering or micrometeringxe2x80x94that is, injecting a very small amount of fuel prior to a main injectionxe2x80x94or post-metering. This is highly desirable in order to improve operation of the fuel injector, especially to reduce noxious emissions and/or to reduce noise of operation. Variable check lift could be advantageous at other times as well. Accurate methods of achieving very small fuel volume pre-metering or micrometering are always of interest.
While it has been proposed in the art that piezoelectric actuators could be employed in fuel injection systems, the use of piezoelectric actuators to directly control needle lift has proven somewhat problematic. First, this is due in part to the fact that only so much space is available within a fuel injector to place a piezoelectric crystal stack. Given the space limitations, the maximum piezoelectric deformation possible in the space available is generally on the order of less than about one hundred microns. Since typical needle valve lifts are on the order of several hundreds of microns, direct piezoelectric control of needle valve lift is not realistic without making substantialxe2x80x94and likely unrealisticxe2x80x94changes in the nozzle area of a fuel injector.
The present invention is directed to addressing these and other concerns associated with controlling needle valve lift within fuel injectors.
In one aspect of the invention, a fuel injector comprises a nozzle at least partially defining a nozzle chamber and at least one injection orifice. A check valve member extends into the nozzle chamber and is slidably disposed in a nozzle body between a first position in which the check valve member obstructs fluid communication between the nozzle chamber and the injection orifice and a second position in which the nozzle chamber and the injection orifice are in fluid communication. A solid state motor in the nozzle body is capable of moving the check valve member toward the second position.
In another aspect of the invention, a method is given for operating a fuel injector having a check valve member slidably disposed in a nozzle body and movable through a range of motion. The range of motion includes a first position in which the check valve member obstructs fluid communication between a nozzle chamber in the nozzle body and at least one orifice in the nozzle body, a second position in which the nozzle chamber and the orifice are in fluid communication, and a third position between the first position and the second position, and substantially closer to the first position than to the second position, in which the check valve member substantially but not entirely restricts fluid communication between the nozzle chamber and the orifice. The method comprises a fuel pressurization step of increasing fuel pressure in the nozzle chamber, a micrometering injection step of operating a solid state motor in the nozzle body to slide the check valve member from the first position to stop at the third position, and a main injection step of increasing fuel pressure in the nozzle chamber to a pressure level sufficient to slide the check valve member in the nozzle body to the second position.